1. Field of the Invention
The present invention relates to a camshaft apparatus, more particularly, to a construction of a thrust bearing supporting a camshaft provided for a V-type double overhead camshaft engine.
2. Description of the Related Art
In a conventional V-type double overhead camshaft engine (referred to as DOHC engine hereinafter), right and left cylinder block-cylinder head assemblies are provided with a camshaft for intake valves and a camshaft for exhaust valves, respectively; i.e., the engine is provided with four camshafts. Accordingly, the number of camshafts is twice that of a usual straight type engine, so that the number of parts of a DOHC engine is larger than a straight type engine.
Japanese Unexamined Patent Publication No. 61-145306 discloses a construction by which common parts are used for camshafts disposed on right and left banks, respectively, to improve a productivity of the engine by reducing the number of parts thereof, and Japanese Unexamined Patent Publication No. 61-232305 shows a construction in which a layout of a driving mechanism for camshafts of V-type DOHC engine is improved so that right and left cylinder heads having the same shape are mounted, and thus the engine becomes more compact.
In camshafts of a V-type engine in which a plurality of intake valves and exhaust valves are synchronously open and closed by two camshafts, and a rotation of one of the camshafts is transmitted to the other camshaft through helical gears, a problem arises of an absorption of a thrust force generated in the other camshaft. Japanese Unexamined Utility Model Publication Nos. 62-110, 62-8424, and 62-8425 disclose a construction of a thrust bearing for a camshaft for resolving this problem. In these publications, a camshaft is formed with an annular thrust flange to define a small clearance between at least one side face of the annular flange and a bearing member, and a hole or groove for supplying lubricating oil to the one side face of the annular flange is formed in the flange or the bearing member. FIG. 1 shows such a construction as disclosed in Japanese Unexamined Utility Model Publication No. 62-8424. In the drawing, a camshaft 50 is rotatably supported in a hole defined by a supporting portion 70 formed on a cylinder head and a bearing cap 60, and flanges 51 and 52 are formed on both sides of the bearing cap 60, so that a thrust bearing portion 80 is defined by the bearing cap 60, the supporting portion 70, and the flanges 51 and 52. Sliding surfaces 81 and 82 on the flanges 52 and 51 face or are in contact with side faces of the bearing cap 60 and the supporting portion 70, and lubricating oil is supplied to one sliding face 81 through oil passages 53 and 71. This publication also describes another construction, in which an oil passage is open to both flanges to supply lubricating oil to both sliding surfaces 81 and 82. In the above constructions, an oil film is always formed on the sliding surfaces, so that abrasion of a bearing surface and noise generated by interference between the camshaft and the bearing surface are prevented.
In a conventional V-type DOHC engine, as understood from FIG. 2, two cylinder heads 11, 21 having the same shape are disposed in parallel to each other in such a manner that a front portion of one of the cylinder heads is disposed next to a rear portion of the other of the cylinder heads, and camshafts are disposed in parallel to each other in such a manner that the front portions of the camshafts of one of the cylinder heads are next to the rear portions of the camshafts of the other cylinder head.
Note, in this description, to simplify the explanation of the layout of the components, the terms "front portion" and "rear portion" are used with respect to the front and rear ends of the vehicle in which the engine is mounted, as denoted by the arrows F and R in FIG. 2.
As shown in FIG. 2, first camshafts 12 and 22 mounted on the cylinder heads 11 and 21 are rotated in the same direction through a belt or a sprocket by the crankshaft of the engine, and second camshafts 13 and 23 are rotated through helical gears 14 and 24 by the rotation of the first camshafts 12 and 22. The gears and members in the bearing portions for the camshafts are employed commonly with respect to the two cylinder heads 11 and 21, as much as possible. The front portions of the camshafts 12 and 13 of the cylinder head 11 are arranged next to the rear portions of the camshafts 22 and 23 of cylinder heads 21, so that a thrust force acts on thrust bearing portions 30 and 31 in the same direction with regard to the second camshafts 13 and 23.
If a groove or hole for supplying lubricating oil to one side of the sliding surfaces as shown in FIG. 1 is provided in the camshafts constructed as described above, in one of the cylinder heads, an oil pressure acts on a side face of the flange (e.g., 51) subjected to a relatively large thrust, and in the other of the cylinder heads, an oil pressure acts on a side face of the flange (e.g., 52) subjected to a relatively low thrust, so that a noise generated by interference between the camshafts and the thrust bearings is not reduced in the other of the cylinder heads. Conversely, if the thrust bearings are constructed in such a manner that the camshafts of the two cylinder heads are subjected to thrust forces acting in the same direction, the thrust bearings in each cylinder have must have a different construction; namely, these thrust bearings can not be commonly used by the two cylinder heads, and thus the cost of assembly is increased.
On the other hand, in a construction in which lubricating oil is supplied to both side faces of the sliding surface of the flange formed on the camshaft, since the camshaft is pressed in both directions by lubricating oil supplied to both side faces, and thus the camshaft is not subjected to a large thrust force, noise generated by interference between the thrust bearings and the camshafts is not satisfactory reduced.